1. Field of the Invention
This invention relates to a steelmaking process in which materials containing iron oxide are directly reduced and the sponge iron is melted by electro-slag resistance process.
2. Discussion of the Prior Art
Residual materials which become available in metallurgy, such as blast furnace dusts, Linz-Donawitz (LD) process dusts and Linz-Donawitz process muds, contain non-ferrous metals or compounds thereof in considerable quantities and for this reason cannot be recharged to the blast furnace or used for steelmaking unless their non-ferrous metal content has been substantially removed.
The non-ferrous metals and their compounds can be removed in a rotary kiln by a reducing treatment with solid carbonaceous reducing agents. As a result of such treatment, the volatilizable non-ferrous metals or their compounds are withdrawn from the rotary kiln in the exhaust gas thereof and can then be separated from said exhaust gases and recovered. The iron oxides are reduced to sponge iron, which is included in the solids discharged from the rotary kiln. Because the non-ferrous metal content of the residual materials virtually always includes Zn or compounds thereof and the volatilization of such Zn content to a high degree and a metallization of substantially all of the iron oxide depends on a surplus of solid carbonaceous material in the rotary kiln, the solids discharged from the rotary kiln contain surplus carbon.
It is also known to use the electro-slag resistance slag-refining process for melting sponge iron and for transforming it to steel ("Stahl und Eisen" 97 (1977), pages 12 to 17). The sponge iron subjected to that known process had been produced by a reduction process in which a gas rather than a solid carbonaceous material is used as a reducing agent. The charging of sponge iron having a high content of surplus carbon, such as is obtained by the direct reduction or the waelz process carried out in a rotary kiln by means of solid carbonaceous reducing agents, would be disturbing because more carbon is present than is required for the final reduction and the surplus varies greatly. Whereas the surplus carbon can be separated from the sponge iron by sieving and magnetic separation and can be separated from the coal ash by flotation or electrostatic separation and be subsequently recycled to the rotary kiln, coal ash becomes available as a residual material in that practice is so fine-grained that it cannot readily be sold. The coal ash also contains certain quantities of carbon and iron, which are lost.
It is the object of the invention to provide for an economical and simple treatment of sponge-iron of sponge-iron containing material discharged from a direct reduction or waelz process carried out in a rotary kiln in which metallurgical residual materials containing iron oxides and volatilizable non-ferrous metals or compounds thereof are treated by means of solid carbonaceous reducing agents--in the electro-slag resistance process.